1. Field of the Invention
This invention relates to an object or target detecting system for a conveyance or craft such as a vehicle, which has an object detector (such as a laser radar) mounted on the vehicle, more particularly to a technique to adjust for the mounting position of the object detector.
2. Description of the Related Art
It was taught by, for example, Japanese Laid-open Patent Application Hei 6-298022 to mount an object detector such as a laser radar (lidar) on a vehicle for detecting an object or target (e.g., another vehicle running ahead on the road) so as to avoid contact therewith.
In carrying out such an object avoidance control, it is needed to accurately detect an object or target existing ahead at a distance, e.g., 100 meters from the vehicle. The object detector such as a laser radar must accordingly be mounted on the vehicle precisely in position. If the vehicle design allows a laser radar to be located at the longitudinal center line (i.e., center in the vehicle width direction) of the vehicle body, it is easy to mount a laser radar precisely in position by aligning the central axis of beam transmitted from the laser radar with the vehicle longitudinal center line.
However, a laser radar is often located at a position offset from the vehicle longitudinal center line for the sake of vehicle design. Accordingly, in a conventional technique to adjust for the radar mounting position, as illustrated in FIG. 12, a target reflector (referred to hereinafter as "target") 100 is placed at a distant position not less than 40 meters ahead of a vehicle 12 and the mounting position of a laser radar 10 is adjusted by aiming the radar 10 at the target 100 such that the central axis of transmitted laser beam 14 is made equal to the direction of the target 100. In the illustrated example, the laser radar 10 is mounted on the vehicle at a position 0.1 meters offset (apart) from a vehicle longitudinal center line 16.
However, it is difficult to prepare such a long range of not less than 40 meters for adjustment of in a factory vehicle assembly line or in a service center. Preferably, the range should be decreased, specifically, to 4 meters to 7 meters, for example, as shown in FIG. 13, or more specifically to 5 meters. However, when the target 100 is placed at the short distance of 5 meters ahead of the vehicle 12 to adjust for the laser radar mounting position by aiming the target 100 such that the central axis of the transmitted beam is made equal to the direction of the target 100, the influence of the offset on the position adjusting (aiming) becomes large.
To be specific, since in the conventional technique the target 100 is placed 40 meters ahead of the vehicle as illustrated in FIG. 12, the aiming error is only 2.5 milliradians (approximately tan.sup.-1 0.1 meter/40 meters). Although this error increases with increasing distance and grows to 0.25 meters (in the vehicle-width direction) at 100 meters, this error of 0.25 meters is still sufficiently small compared to the standard width of traffic lane (i.e., 3.5 meters) and can accordingly be neglected.
On the other hand, if the checking range is shortened to 5 meters, the aiming error increases to 20 milliradians (approximately tan.sup.-1 0.1 meters/5 meters). The error grows to 2.0 meters (in the vehicle-width direction) at 100 meters, which may sometimes result in a problem of misdetecting a vehicle in a next lane as an object or target.